This invention relates generally to integrated circuit processes and fabrication, and more particularly, to a precursor and synthesis method, having a substituted phenylethylene ligand, such as .alpha.-methylstyrene, which improves liquid phase stability, and which is capable of depositing copper at high deposition rates, low resistivity, and with good adhesion on selected integrated circuit surfaces.
The demand for progressively smaller, less expensive, and more powerful electronic products, in turn, fuels the need for smaller geometry integrated circuits (ICs) on larger substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the area of the interconnecting surfaces is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the operation of high speed ICs. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different impedance characteristics.
There is a need for interconnects and vias to have both low resistivity, and the ability to withstand process environments of volatile ingredients. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals are popular because they are easy to use in a production environment, unlike copper which requires special handling.
Copper (Cu) would appear to be a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having nearly half the width of an aluminum line.
The electromigration characteristics of copper are also much superior to those of aluminum. Aluminum is approximately ten times more susceptible than copper to degradation and breakage due to electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity.
There have been problems associated with the use of copper, however, in IC processing. Copper pollutes many of the materials used in IC processes and, therefore barriers are typically erected to prevent copper from migrating. Elements of copper migrating into these semiconductor regions can dramatically alter the conduction characteristics of associated transistors. Another problem with the use of copper is the relatively high temperature needed to deposit it on, or removing it from, an IC surface. These high temperatures can damage associated IC structures and photoresist masks.
It is also a problem to deposit copper onto a substrate, or in a via hole, using the conventional processes for the deposition of aluminum when the geometries of the selected IC features are small. That is, new deposition processes have been developed for use with copper, instead of aluminum, in the lines and interconnects of an IC interlevel dielectric. It is impractical to sputter metal, either aluminum or copper, to fill small diameter vias, since the gap filling capability is poor. To deposit copper, first, a physical vapor deposition (PVD), and then, a chemical vapor deposition (CVD) technique, have been developed by the industry.
With the PVD technique, an IC surface is exposed to a copper vapor, and copper is caused to condense on the surfaces. The technique is not selective with regard to surfaces. When copper is to be deposited on a metallic surface, adjoining non-conductive surfaces must either be masked or etched clean in a subsequent process step. As mentioned earlier, photoresist masks and some other adjoining IC structures are potentially damaged at the high temperatures at which copper is processed. The CVD technique is an improvement over PVD because it is more selective as to which surfaces copper is deposited on. The CVD technique is selective because it is designed to rely on a chemical reaction between the metallic surface and the copper vapor to cause the deposition of copper on the metallic surface.
In a typical CVD process, copper is combined with a ligand, or organic compound, to help insure that the copper compound becomes volatile, and eventually decomposes, at consistent temperatures. That is, copper becomes an element in a compound that is vaporized into a gas, and later deposited as a solid when the gas decomposes. Selected surfaces of an integrated circuit, such as diffusion barrier material, are exposed to the copper gas, or precursor, in an elevated temperature environment. When the copper gas compound decomposes, copper is left behind on the selected surface. Several copper gas compounds are available for use with the CVD process. It is generally accepted that the configuration of the copper gas compound, at least partially, affects the ability of the copper to be deposited on to the selected surface.
Copper metal thin films have been prepared via chemical vapor deposition by using many different kinds of copper precursors. In 1990, D. B. Beach et al. Chem. Mater. (2) 216 (1990) obtained pure copper films via CVD by using (.eta..sup.5 --C.sub.5 H.sub.5)Cu(PMe.sub.3), and later, in 1992, H. K. Shin et al., Chem. Mater. (4) 788 (1992) declared the same results by using (hfac)Cu(PR.sub.3).sub.n (R=methyl and ethyl and n=1 and 2). However, these copper precursors are solids, which can not be used in the liquid delivery system for copper thin film CVD processing. Furthermore, the copper films often contain large amounts of carbon and phosphorus contamination, which can not be used as interconnectors in ICs.
Cu.sup.2+ (hfac).sub.2, or copper (II) hexafluoroacetylacetonate, precursors have previously been used to apply CVD copper to IC substrates and surfaces. However, these Cu.sup.2+ precursors are notable for leaving contaminates in the deposited copper, and for the relatively high temperatures that must be used to decompose the precursor into copper.
Earlier studies of copper precursors concentrated on the evaluation of a series of copper(I) fluorinated .beta.-diketonate complexes, which have been proven to be very promising sources for the use in the chemical vapor deposition of copper metal thin films. Copper(I) fluorinated .beta.-diketonate complexes were first synthesized by Gerald Doyle, U.S. Pat. No. 4,385,005 (1983) and 4,425,281 (1984), in which he presented the synthesis method and their application in the separation of unsaturated organic hydrocarbons. In the U.S. Pat. No. 5,096,737 (1992), Thomas H. Baum, et at., claimed the application of these copper(I) fluorinated .beta.-diketonate complexes as copper precursors for CVD copper thin film preparation. Copper thin films have been prepared via chemical vapor deposition using these precursors.
Among several liquid copper precursors, 1,5-dimethyl 1,5-cyclooctadiene copper(I) hexafluoroacetylacetonate mixed with 1,6-dimethyl 1,5-cyclooctadiene copper(I) hexafluoroacetylacetonate ((DMCOD)Cu(hfac)) and hexyne copper(I) hexafluoroacetylacetonate ((HYN)Cu(hfac) were evaluated in detail. The copper thin films deposited using (DMCOD)Cu(hfac) have very good adhesion to metal or metal nitride substrates, but a high resistivity (2.5 .mu..OMEGA..multidot.cm) and a low deposition rate. (HYN)Cu(hfac) copper film has poor adhesion to a TiN substrate, and high resistivity (.about.2.1 .mu..OMEGA..multidot.cm). Another compound, butyne copper(I)(hfac), ((BUY)Cu(hfac)), gives a copper film with low resistivity (1.93 .mu..OMEGA..multidot.cm), but has poor adhesion and is relatively expensive. Also, the compound is a solid and, therefore, difficult to use in a liquid delivery system. The invention of copper(I)(hfac) stabilized with a series of trialkylvinylsilane (John A. T. Norman et al., U.S. Pat. No. 5,085,731 (1992)) improved the properties of copper thin films.
Copper films deposited using a liquid copper precursor, (hfac)Cu(TMVS), where TMVS=trimethylvinylsilane, have low resistivities and reasonable adhesion to substrates. This precursor is useful because it can be used at relatively low temperatures, approximately 200.degree. C. This liquid copper precursor has been used for the preparation of copper metal thin films via CVD for some time, but there are still some drawbacks: stability, the adhesion of copper films, and cost for the trimethylvinylsilane stabilizer. Also, the precursor is not especially stable, and can have a relatively short shelf life if not refrigerated. Various ingredients have been added to (hfac)Cu(tmvs) to improve its adhesiveness, temperature stability, and the rate at which it can be deposited on an IC surface. U.S. Pat. No. 5,744,192, entitled "Method Of Using Water To Increase The Conductivity Of Copper Deposited With Cu(HFAC)TMVS", invented by Nguyen et al., discloses a precursor and method of improving the electrical conductivity of Cu deposited with (hfac)Cu(tmvs).
It is generally acknowledged in the industry that (hfac)Cu(tmvs) becomes unstable, and begins to decompose, above 35.degree. C. Use of a (hfac)Cu(tmvs) precursor stored at this temperature leads to undesirable process results. The effectivity of (hfac)Cu(tmvs) stored at temperatures lower than 35.degree. C. is also unpredictable. A "fresh" batch of precursor, or precursor stored at temperatures well below room temperature, is used to guarantee predictable processes.
A Cu precursor comprising a substituted phenylethylene, and synthesis method for same, is disclosed in co-pending U.S. Ser. No. 09/210,099, entitled "Substituted Phenylethylene Precursor and Synthesis Method", invented by Zhuang et al. The above-mentioned application is incorporated by reference herein.
It would be advantageous if a copper precursor was found that effectively deposits copper with low resistivity and good adhesion properties. It would be further advantageous if this precursor was inexpensive to synthesize.
It would be advantageous if a Cu precursor could be deposited on an IC wafer with great adhesion, to permit the deposition of a subsequent Cu film at high deposition rates.
Accordingly, a method for depositing a copper (Cu) seed layer on an integrated circuit (IC) wafer is provided comprising the steps of:
a) volatizing a Cu precursor compound including Cu.sup.+1 (hexafluoroacetylacetonate), and a substituted phenylethylene ligand including one phenyl group bonded to a first carbon atom, with the remaining bond to said first carbon atom being selected from a first group consisting of C.sub.1 to C.sub.6 alkyl, C.sub.1 to C.sub.6 haloalkyl, phenyl, and C.sub.1 to C.sub.6 alkoxyl, and in which a second carbon atom includes a second and third bond, said second and third bonds being selected from the group consisting of H, C.sub.1 to C.sub.6 alkyl, phenyl, and C.sub.1 to C.sub.6 alkoxyl, preferably, the substituted phenylethylene ligand is .alpha.-methylstyrene; PA1 b) decomposing the Cu precursor compound on a Cu-receiving surface of the IC wafer to form a Cu seed-layer, whereby the seed layer is formed through chemical vapor deposition (CVD); and PA1 c) forming a second layer of Cu overlying the Cu seed layer, whereby the Cu seed layer enhances adhesion between the Cu films and Cu-receiving surface.
In some aspects of the invention the Cu precursor compound includes an additive to create a Cu precursor blend. Then, the Cu precursor blend further includes an additive of approximately 15%, or less, substituted phenylethylene, as measured by weight ratio of the Cu precursor compound. When Step a) includes the substituted phenylethylene ligand being .alpha.-methylstyrene, then the substituted phenylethylene ligand in the additive is also .alpha.-methylstyrene.
Once the seed layer is in place, Step c) includes depositing the second Cu layer with deposition methods selected from the group consisting of CVD, physical vapor deposition (PVD), and electroplating. An .alpha.-methylstyrene Cu precursor could even be used to form a relatively thick second Cu layer, as compared to the Cu seed layer thickness, at high deposition rates.